1. Field of the Invention
The present invention relates generally to a portable food warming appliance and methods employing such appliances for heating foods for consumption by an individual or large numbers of people.
2. Prior Art
Many proposals have been made for incorporating food warming devices into portable containers, such as lunch boxes and the like, for transport together with prepared food and such utensils as are required to serve and consume the food. Proposals have also been made in an effort to provide portable lunch kits specially adapted to receive, transport, and heat frozen food trays. One such proposal is set forth in U.S. Pat. No. 3,385,952, issued May 28, 1968 to E. W. Mix.
A significant disadvantage of known portable food warming devices is the inefficiency with which they heat food. Portable food warmers to date have principally relied on the heating techniques of convection and radiation to warm the food. Typically one or more heating devices is disposed in spaced relationship with the food to be warmed. While a number of proposed food warmers dispose the heating element within a chamber which also holds the food to be warmed, most dispose the heating element outside the chamber. In either arrangement, the resulting system is in essence a small oven providing a heating compartment which is substantially larger than the food positioned therein for warming. Very little use has been made in known portable food warmers of the more efficient heating technique of conduction as opposed to radiation and convection.
Radiation and convection heating techniques are not only less efficient and consequently more costly than is conduction heating, but they further necessitate the heating of one or more heating elements to a temperature which is higher than that needed for conduction heating. To express this problem in another way, radiation and convection techniques require more time to transfer a given quantum of heat to the food than does conduction heating, assuming the heating elements in both situations are at the same temperature. Accordingly, in order to warm the food within a reasonable period of time by radiation and convection techniques, the heating element temperature is ordinarily elevated considerably above the temperature to which the food is to be heated.
High temperature heating element operation, in and of itself, creates several problems. First, the food warmer components must be constructed to withstand the high heating element temperatures. This often necessitates the use of special heat resistant materials thereby increasing the cost of the food warmer. Second, where high heating element temperatures are involved, substantial amounts of heat insulating materials must be provided to keep the exterior portions of the food warmer case cool.
Still other problems due to high heating element temperatures are encountered with respect to the warming function of the apparatus in that the foods being warmed often tend to scorch and dry out. The food portions nearest the heating element tend to reach temperatures above the temperature to which the food is to be warmed while interior portions of the foods are still below the desired temperature. Consequently, depending on the type of food being warmed, various adverse surface effects are encountered. The surfaces of some foods tend to "cook" rather than to "warm." This changes the character of the food being warmed in an undesirable fashion. The surfaces of other foods tend to dry out so as to become undesirably crusty. Other foods tend to scorch or burn giving them a very unpalatable appearance and flavor.
Most known food warming devices cannot be used to keep food warm and ready to serve for a prolonged period of time without adversely affecting the food. This disadvantage is particularly annoying where the operator has energized the food warmer in anticipation of consuming the food at a predetermined future time, and is then delayed unexpectedly whereupon the food becomes overheated and cooks, scorches, dries out, burns or otherwise loses its appetizing appeal. It is not uncommon that the food is rendered entirely inedible where it is left in the heated apparatus substantially longer than anticipated.
There are many industries where large shifts of employees are given brief lunch breaks that begin and end at set periods. While it is desirable to provide each employee with a warm lunch, significant problems and expenses are encountered in maintaining and operating cafeterias of sufficient size to accommodate large numbers of employees in short time intervals. Moreover, a number of employees find they can bring food from home on a daily basis at a lesser cost than is involved in cafeteria dining. Still other employees are faced with special dietary requirements that cannot be provided for in the cafeteria menu.
In other industries, for instance construction work, there are no cafeterias or other facilities where the employee can readily purchase a warm lunch. In these employments, it is common for the workers to carry their lunches daily from home. If the employee wants part or all of his lunch warm at his lunch break, the usual approach is to pre-heat the food at home and pack it in insulated containers. While the insulated containers have been remarkably improved in recent years, the fact remains that they do nothing in and of themselves to further heat the food, and the food accordingly gradually cools off. Moreover, many foods deteriorate if kept at elevated temperatures for several consecutive hours. Accordingly, the types of foods which can be kept warm are substantially limited in number.
Accordingly, while it is recognized as being desirable to provide employees with facilities which will enable them to daily enjoy a warm lunch including food of their choice at a minimum of expense to the employee, the fact remains that there is no way to provide cafeteria-style facilities for employees in many industries, and where cafeterias are provided they still do not serve the needs of some employees.
While many portable lunch kits with self contained heating units and control timers have been proposed as a possible solution to the workman's need for a warm lunch, the fact remains that the proposed lunch kits have had too many problems associated therewith to gain any substantial acceptance.